Rocket reuse became a normal occurance, probably earlier than the majority of the industry anticipated. Their competitive pricing took the market by storm, changing the equation of sending anything to space. There are less new payloads to launch in 2019, because it takes much longer to contract and build a satellite than to send it to space, and the market hasn't yet adapted to this new mechanic.

Their need for manufacturing new boosters scaled down greatly because of reusability. You cannot reasign all engineers to other projects, some must go.

On top of that SpaceX is moving to new risky projects like the Starship, and they need to cut any fat that poses risk to their long term plans.

Lastly, as few pointed out, it's a great opportunity to get rid of underperforming employees and restructure the company.